Myostatin (or GDF-8) is a negative regulator of muscle growth and is structurally related to the transforming growth factor β (TGF-β) superfamily (McPherron et al 1997a). More particularly, myostatin is a potent negative regulator of skeletal muscle during development, and in adult life, in a wide range of species from fish to mammals (McPherron and Lee, 1997). Myostatin is known to regulate its own expression via a mechanism that is incompletely understood at present (Berry et al. 2002, Spiller et al. 2002, Rebbapragada et al. 2003).
The myostatin protein is initially translated as a 375 amino acid precursor molecule having a secretory signal sequence at the N-terminus, a proteolytic processing signal (RSRR) of the furin endoprotease, and nine conserved cysteine residues in the C-terminal region to facilitate the formation of a “cysteine knot” structure. Myostatin is activated by furin endoprotease cleavage at Arg 266 releasing the N-terminal, or “latency-associated peptide” (LAP) and the mature, C-terminal domain, which dimerises to form the active myostatin molecule. After processing, a homodimer of the LAP peptide remains non-covalently bound to the homodimer of mature myostatin in an inactive complex (Lee et al. 2001). Other proteins, for example, follistatin, titin cap, GDFP1, follistatin related gene and hSGT are also known to bind to and regulate the secretion and activation of the latent myostatin complex (Lee et al. 2001, Nicolas et al. 2002, Hill et al. 2002, Hill et al. 2003, Wang et al. 2003).
It has been demonstrated that myostatin inhibits myoblast proliferation and differentiation without inducing apoptosis or stimulating muscle protein breakdown (Thomas et al. 2000, Langley et al. 2002, Rios et al. 2001, Taylor et al. 2001). Knock-out mice for myostatin have greatly increased muscle mass over their entire body. Myostatin-null mice have approximately 30% greater body weight than normal mice, and exhibit a 2-3-fold increase in individual muscle weights due to muscle fibre hyperplasia and hypertrophy. Natural mutations in myostatin have been identified as being responsible for the “double-muscled” phenotype, such as the Belgian Blue and Piedmontese cattle breeds (McPherron et al 1997b, Kambadur et. al. 1997, Grobet et al. 1997).
Myostatin has also been linked with many other biological processes. For example, knockout transgenic mice have altered cortical bone structure indicating a role in osteogenesis (Hamrick 2003). Furthermore, myostatin has been shown to be involved in regulating glucose and fat metabolism, thus it may be implicated in type 2 diabetes and obesity (McPherron and Lee 2002).
In accordance with these effects, myostatin has been implicated in a number of disorders associated with muscle wasting, or muscle atrophy, such as that seen in individuals affected by HIV, cancer, prolonged bed rest, or muscular dystrophy (Gonzalez-Cadavid et al. 1998, Langley et al 2004, Zachwieja et al 1999, Bogdanovich et al. 2002). It was demonstrated that in vivo administration of myostatin induces cachexia, a severe form of muscle wasting associated with cancer and sepsis (Zimmers et al. 2002). Furthermore, up-regulation of myostatin in glucocorticoid-induced muscle atrophy has been observed (Ma et al. 2003). Changes in myostatin expression have been shown in other conditions, for example, up-regulated in cardiomyocytes after heart damage, and down regulated in regenerating muscle (Sharma et al. 1999).
Despite the available information, complexity in the molecular regulation of biological processes by products of the myostatin gene remains incompletely understood. Given the role of myostatin in regulation of muscle growth and differentiation, tissue regeneration, clearly there is a need for improved compositions and methods to intervene in these and other processes in which myostatin plays a role. The present invention fulfils these needs, in part by providing a novel, biologically active myostatin splice variant, and also offers other related advantages.